The present invention relates to the field of current regulation for electrical devices, and in particular to a method and apparatus for regulating current in a voltage-injecting converter wherein the converter is capable of automatically switching between two-component current regulation and active current regulation in response to operating conditions. According to the invention, a flux-forming current component and a torque-forming current component of an actual current vector are determined and regulated to a current component of a desired current vector, with a precontrol value superimposed over each controlled variable.
Polyphase machines with regulated drives may require the injection of desired currents into the machine. Where this current is provided by a converter with a desired voltage value input, such as a pulse converter or direct converter type of voltage-injecting converter, the required voltages must be determined so that the desired currents appear in the machine. The dynamics of the drive are therefore dependent on how well the actual current value conforms to the required current value.
A required current may be generated in several ways. For example, "IEEE Transactions on Industry Applications," vol. IA-16, no. 2, 186-192 (March/April 1980) and "IEEE Transactions on Industry Applications," vol. IA-22, no. 4, 691-696 (July/August 1986) disclose a current regulation method which uses two current regulators to control two components of a current vector to desired values. The current regulators are supported by a pilot control which calculates the voltages required by the machine, although the current regulators must also output voltages which are not determined by the pilot control (e.g., dynamic components, errors, etc.). The currents are advantageously regulated in a field-oriented system, because in such a system of coordinates the currents are easily-regulated direct currents and the required voltages for pilot control of the current regulators can easily be calculated. This regulating method is referred to as two-component current regulation, and is characterized by very good dynamic behavior, especially in low and medium RPM ranges.
A disadvantage of two-component regulation arises from coupling between the two regulating circuits, a phenomenon that steadily increases with RPM. This coupling can appear as a disturbance in slow discharge processes having an unbalanced precontrol network, especially at low regulator amplifications. Moreover, two-component current regulation with high modulation can only operate when a voltage reserve is available. Since a blind current component (as opposed to a flux) is injected, the drive reacts with great sensitivity to an improper setting of the main inductance. As a rule, therefore, superimposed flux regulation is required in such devices.
A further disadvantage of two-component regulation is that its stability depends on the frequency and regulating amplification of the current regulator. The higher the frequency or the smaller the amplification, the poorer will be the damping of the regulation; however, it is precisely at such higher frequencies that control amplification must be withdrawn due to reduced dynamics of the rectifier. Another difficulty arises in field-weakening operation. Here, the field must be weakened in two-component current regulation to the point where a certain control reserve is still available, thus reducing rectifier utilization.
A second type of current regulation, known as active current regulation, is disclosed in "RPM Regulation Around Zero," Journal Elektrotechnik, vol. 74, no. 7/8, 24-31 (Aug. 21, 1992). Here, as in two-component current regulation using a pilot control, the required voltage value is determined in field-oriented coordinates. Unlike two-component current regulation, the current is only regulated in the torque-producing direction through a frequency change in the required voltage regulator. The second current component, the magnetization current, adjusts itself. Thus, only one of the two current components of a motor current in the field-oriented system of coordinates is regulated. Stator frequency is manipulated to accomplish this regulation, with the components of the stator voltage following exclusively from a precontrol network.
If the control limit of the rectifier is reached in this case, the active current regulator is still able to regulate the active current to a certain value through a frequency change. A superimposed voltage maximum regulation feeds the precontrol network by way of approximation with the magnetization current actually flowing in the machine. The transition from basic RPM to field-weakening operation is accomplished without difficulty, enabling use of the maximum control range. A superimposed voltage maximum regulation matches, within the field weakness range, the magnetization current required value for the precontrol network.
The advantages of active current regulation lie in its performance at high-rotational speeds and its high degree of control. Moreover, active current regulation requires no voltage reserve, and the transition to field-weakening operation occurs smoothly. Also, since voltage is calculated and injected depending on a desired flux, no superimposed flux regulation is required.
On the other hand, active current regulation has certain disadvantages compared to two-component current regulation. Because the current regulator influences only the frequency of the required voltage value, active current regulation exhibits only a slight dynamic. Another difficulty arises at low frequencies, where the length and position of the voltage setpoint indicator is such that the desired change in the active current cannot be produced by the active current regulator through a change in frequency. In other words, active current regulation is inoperable below a minimum frequency.